It is known from PCT International Application No. WO 90/15625 to conduct radioimmunotherapy using actinium-225, or one of its daughters, as the alpha-particle source in a radioimmunoconjugate which also comprises a chelating agent and a tumorspecific antibody. In addressing some of the disadvantages of previously proposed alpha-particle emitters, this patent application identifies radon-220, a highly radioactive inert (noble) gas from the decay chain of thorium-228 and radium-224, as an undesired by-product of producing bismuth-212, for example. The occurrence of radon-220 in a decay chain requires special shielding and containment facilities if used in a hospital setting for example.
The above-mentioned patent application teaches the use of uranium-233, which is relatively abundantly stockpiled as an unused source of fuel for nuclear breeder reactors. The use of this material as a source for the actinium-225, through its intermediate decay product thorium-229, is said to possess a number of advantages, including the production of no significant amounts of noble gas isotopes, such as radon-220.
Unfortunately, the availability of uranium-233 as a starting material for the manufacture of the desired intermediate product, thorium-229, as a source for the actinium-225 may be quite limited due to governmental security regulations, on the one hand, and a potentially largely increased demand for this material, on the other hand. Hence, there is a need for non-controlled alternative source materials of greater availability.
The use of radium-226 will cope with the availability issue. Its irradiation by neutrons in a nuclear high flux reactor will generate the needed thorium-229. But the co-generation of comparable amounts of thorium-228 causes the occurrence of radon-220 as one of the decay products in this product mix. Both thorium isotopes decay in a relatively large cascade, starting with radium isotope, being radium-225 from thorium-229 and radium-224 from thorium-228. It is the decay train of undesired radium-224 which is responsible for the production of the undesired radon-220.